Polymer based composite materials consisting of reinforcing fibers and a matrix resin are widely used for sporting goods, aerospace, general industries, etc. Fiber reinforced composite materials are produced by various methods, and among the methods, it is widely practiced to use intermediate sheets, in which reinforcing fibers are impregnated with an unhardened matrix resin, as a prepreg. In this method, usually, sheets of a prepreg are laminated and heated, to form a composite material. The matrix resins used for prepregs include both thermosetting resins and thermoplastic resins, and in most cases, thermosetting resins excellent in handling convenience are used. Above all, epoxy resins are used most frequently. Furthermore, maleimide resins, cyanate resins, and their combinations are also often used.
In general, polymer based materials decline in strength and elastic modulus at high temperature and/or high humidity. Therefore, fiber reinforced composite materials using a polymer as a matrix are also likely to decline in physical properties such as strength at high temperature or high humidity. However, the composite materials used as structural materials of airplanes, vehicles, ships, etc. are required to keep physical properties sufficiently even at high temperature and/or high humidity. When a fiber reinforced composite material is used as a structural material, compressive strength is an especially important physical property. The compressive strength can be measured by using a unnotched coupon, notched coupon or cylinder, etc. as a test piece, but in actual use, since the fiber reinforced composite material is often used as a plate with bolt holes, the compressive strength of a notched coupon especially at high temperature and at high humidity is important.
However, conventional polymer based composite materials are not always sufficient in the compressive strength at high temperature or high humidity even though they are advantageously light in weight. So, they are sometimes limited in applicability. To enhance the compressive strength at high temperature and high humidity, it is effective to enhance the elastic modulus of the resin, and it is considered important to prevent the decline of elastic modulus at high temperature and high humidity. For enhancing the elastic modulus of the resin, it has been proposed to use epoxy resins higher in crosslinking density, and for preventing the decline of elastic modulus at high temperature and high humidity, it has been proposed to decrease the percentage of water absorption and to introduce a heat resistant skeleton. As resin compositions for prepregs balanced in impact resistance, heat resistance and water resistance, Japanese Patent Laid-Open Nos. 62-297316 and 62-297312 disclose resin compositions consisting of an epoxy resin composition mainly composed of an epoxy resin with triglycidylaminophenol skeleton, and diaminodiphenylsulfone (DDS), polyether sulfone (PES) or polyether imide (PEI).
However, their heat resistance is evaluated in reference to glass transition temperature, and water resistance, to the percentage of water absorption, respectively separately, and the problem that an epoxy resin with triglycidylaminophenol skeleton declines greatly in physical properties at high temperature and high humidity is not solved at all. Furthermore, to prevent the decline of hardened epoxy resins in physical properties caused by moisture absorption, less hygroscopic diamine hardening agents and their epoxy resin compositions are disclosed in Japanese Patent Laid-Open Nos. 59-215314, 59-215315 and 60-67526. It is disclosed that if any of the special diamine hardening agents is used, the hardened epoxy resin composition keeps high physical properties even at high temperature and high humidity, and has high impact resistance as a non-interlayer toughened prepreg.
However, if an actual application as a prepreg is considered, the amount of the special curing agent added to the epoxy resin must be larger than the amounts of the conventional curing agents, to obtain sufficient physical properties, since the special curing agent is large in molecular weight compared with the number of reaction points (the number of amine hydrogen atoms). For this reason, the viscosity of the composition becomes high, and as a result, the degree of freedom in resin formulation is significantly limited. For example, if it is attempted to enhance impact resistance by adding a thermoplastic resin or using an interlayer toughening technology for meeting the demand for higher impact resistance, the viscosity of the resin increases. So, the application of such a technology is considered to be practically impossible, and it is difficult to enhance the impact resistance higher than the disclosed level.
As a resin composition excellent in heat resistance and flow property during resin curing, Japanese Patent Publication No. 7-78138 describes to combine 3,3'-DDS and a thermoplastic resin of 100.degree. C. or higher in Tg with epoxy resins, and also states that if heat resistance is especially important, it is preferable to let the content of tetraglycidyldiaminodiphenylmethane (TGDDM) account for 50 to 80% of all the epoxy resins. The main effect found by that invention is that if 3,3'-DDS high in the solubility in epoxy resins and high in reactivity is used instead of 4,4'-DDS, the reaction with the epoxy resins during heating for curing can be expedited to allow the minimum viscosity to be kept high. If an epoxy resin is cured by using 3,3'-DDS like this, the cured resin can be high in heat resistance, but on the other hand, the toughness of the resin decreases, to also decrease impact resistance. The above invention does not solve this problem at all.
As a prepreg excellent in impact resistance, Japanese Patent Publication No. 6-94515 discloses a particulate interlayer toughening technology of adding fine thermoplastic resin particles to a thermosetting resin used as a base resin. An example described in the patent gazette discloses a composition containing TGDDM by 90% in the epoxy resins as a thermosetting resin used as a base resin, 4,4'-DDS by 0.175 time the mole number of epoxy groups, and 10% of PES as a thermoplastic resin. The invention disclosed above is mainly intended to provide a technology for enhancing the toughness of a laminate selectively at its interlayer regions where stresses are concentrated most under impact, by adding a thermoplastic resin. The invention discloses a technology concerned with impact resistance, but does not solve anything about improvement of compressive strength. Japanese Patent Laid-Open Nos. 5-1159 and 4-268361 also disclose interlayer toughening technology, and describes cases of using an epoxy resin consisting of TGDDM and triglycidylaminophenol as a base resin, 3,3'-DDS as a curing agent, and a polysulfone or a PES oligomer with amine terminals as a thermoplastic resin. These inventions are also mainly intended to improve impact resistance by interlayer toughening. These resin compositions are improved in impact resistance as disclosed in the examples, but since triglycidylaminophenol used accounts for 40 to 50% of all the epoxy resins, the elastic modulus of the resin at high temperature and high humidity is not so high, and hence the compression property at high temperature and high humidity is not sufficient either.